Aqueous coating materials comprise binders which are either soluble in or dilutable in water; the water-dilutable binders include resin components and resins which have been emulsified or rendered self-emulsifiable, and aqueous dispersions. Examples of water-soluble binders are modified alkyd resins, acrylate resins, linear and branched saturated polyester resins, epoxy resins, shellac and modified phenolic resins. Examples of water-dilutable binders are--optionally modified--dispersed or water-dispersible, unsaturated and saturated polyester resins, alkyd resin emulsions, polyurethane dispersions, one-component epoxy resin emulsions, emulsifiable two-component epoxy resin systems, dispersible alkyd, phenolic, melamine and urea resins, (meth)acrylate dispersions, (meth)acrylate copolymer dispersions, polystyrene and styrene copolymer dispersions, poly(vinyl ether) dispersions, and polyvinyl chloride and polyvinylidene chloride dispersions.
In addition to water-soluble or water-dilutable binders, aqueous coating materials generally include pigments, fillers, water-soluble crosslinkers and crosslinking catalysts, and further additives such as, for example, levelling assistants, devolatilizing assistants and wetting agents.
These compositions are applied in aqueous form, for example to metallic substrates, mineral substrates, plastics, wood, paper or glass. Following application, the aqueous film dries to a solid coating film. Films which dry at ambient temperature (room temperature) after drying in air, are optionally subjected to thermal conditioning/post-drying at temperatures up to 100.degree. C. For other types of aqueous coating material, even higher temperatures (120-220.degree. C.) may be necessary for film formation, if the latter requires the chemical reaction (post-crosslinking) of two or more binder components.
The water-soluble and water-dilutable binders listed predominantly comprise thermoplastic or thermosetting polymers, which are generally hard but brittle. To improve the film properties, therefore, impact modification of the polymer systems is frequently necessary. Silicones are known as modifiers for thermoplastics and thermosets and are of particular interest since they not only increase the impact strength but also improve low-temperature flexibility, weathering stability, stability to fluctuating temperature stress and chemical resistance of thermoplastic or thermosetting organic-polymer systems. Generally disadvantageous, however, is the fundamental incompatibility of silicones with organic polymers. The incompatibility may induce flow defects in the course of use, while migration of the silicones gives rise to adhesion problems and problems associated with overcoatability.
In DE-A 3922285 (=GB-A 2222167) (Dow Corning, laid open on Jan. 25, 1990) the known incompatibility of siloxanes with organic polymers is exploited for decorative purposes; it describes a coating composition composed of polydiorganosiloxane and of an aqueous dispersion of a film-forming polymeric material, for obtaining a hammered surface effect. Polysiloxanes of this kind are not suitable for the impact modification of aqueous coating materials, since the polysiloxane is present in emulsified form as a liquid in the water phase rather than as discrete particles. In the amount which needs to be used for impact modification, these compounds are exuded from the coating film.
EP-A 586048 (Shin Etsu, published on Mar. 9, 1994) describes diol-functional silicone oils as additives to aqueous coating materials for lowering the surface tension and improving the substrate wettability; in this established utility, these siloxanes show better flow properties than polyether-modified silicone oils. Siloxanes of this kind, present in emulsified form, are unsuitable for impact modification. The document goes on to describe that in amounts employed of &gt;5% by weight there is a decrease in the mechanical strength of the polymer film modified with these compounds.
WO-A 93/14169 (Crompton Garland Ltd., published on Jul. 22, 1993) describes a formulation of a crosslinkable aqueous phenolic resin emulsion and of a silicone resin emulsion. In the course of the crosslinking of the phenolic resin component there is chemical attachment of the silicone to the phenolic resin, as a result of which the silicone component becomes included in the phenolic resin matrix as a plasticizer. U.S. Pat No. 4,803,233 (Dow Corning, granted on Feb. 7, 1989) describes aqueous mixtures which comprise organic polymer, silicone resin and a combination of three different nonionic emulsifiers. Depending on the polymer system and silicone component, the addition of silicone resin brings about--in some cases only after thermal treatment of the homogeneous blends--improved tensile strength, adhesion and corrosion resistance of the coating materials.
An improvement in the impact strength coupled with retention of hardness is described neither in WO-A 93/14169 nor in U.S. Pat. No. 4,803,233. There is no controlled microphase separation therein, since the substances involved are not organic-polymer-compatible modifiers with a particulate structure.
EP-A 541395 (Takemoto, published on May 5, 1993) describes aqueous coating compositions for thermoplastic polyester films on the basis of aqueous emulsions of polysiloxane-polyvinyl graft copolymers. These aqueous binder systems improve surface properties of the polyester films, such as smoothness, release properties, and water- and oil-repellency properties. In this case the graft copolymers are not employed as modifiers for organic-polymer binders in aqueous coating materials but are used as an aqueous coating composition with which it is possible to obtain particular surface properties in the context of application to thermoplastic polymer films.
WO-A 90/08810 (ICI Australia Operations Proprietary Ltd., published on Aug. 9, 1990) describes aqueous coating compositions on the basis of silicone-containing polymers, so-called "multi-polymer" particles. The "multi-polymer particles" are prepared by polymerizing the monomers in a mixture with silicone polymer. In this case the silicone constituent must be added to the monomer/monomers prior to polymer preparation and cannot be added as a formulation constituent to (just any) aqueous binder systems. The silicone component in the water-dilutable binder systems that are obtained in this case enhances, for example, the UV stability of the coating materials; an improvement in the impact strength is not described. Again, there is no controlled microphase separation in this case, since the silicone components employed are liquids or resins which do not comprise organic-polymer-compatible modifiers with a particulate structure.